1. Field of the Invention
The present invention relates to a lens driving apparatus for moving a lens, such as a focus lens for carrying out focusing in particular, along an optical axis.
2. Related Background Art
FIG. 6A is an overall view of the lens driving apparatus disclosed in Japanese Patent Application Laid-Open No HEI 9-96753. This lens driving apparatus 101 comprises a driving source 102 having a lead screw 111; and a lens holding frame 103, provided with a rack part 113 in mesh with the lead screw 111, for holding a lens 110. When the driving source 102 is driven, the lens 110 is moved along the optical axis O. In this lens driving apparatus 101, the engagement between the lead screw 111 and the rack part 113 yields a backlash along the depicted optical axis O, whereby there have been cases where the movement of the lens 10 is retarded by the amount of backlash when the movement of the lens holding frame 103 is reversed, for example, which may adversely affect the positioning accuracy of the lens 110. A lens driving apparatus having a similar configuration is also disclosed in Japanese Patent Application Laid-Open No. 2000-275,194.
FIG. 6B is a sectional view of another conventional lens driving apparatus. In this lens driving apparatus 201, a lens holding frame 203 is urged along the optical axis O by a spring 220. As a consequence, the lens holding frame 203 is always biased in one direction by the amount of backlash, so that there are no xe2x80x9crattles,xe2x80x9d and the problem mentioned above is overcome as well.
In the conventional lens driving apparatus 201 shown in FIG. 6B, however, one end of the spring 220 is secured to a fixing end face 215, whereby the force urging the lens holding frame 203 caused by the spring 220 varies depending on positions of the lens holding frame 203. For example, the urging action caused by the spring 220 has become stronger in a position closer to the fixing end face 215. In such a case, the biasing action caused by the lens holding frame 211 with respect to the lead screw 211 becomes stronger, so that a stronger load is imposed on the rotation of the driving source 202, which may prevent the positioning of the lens 210 from being accurately controlled and lower the positioning accuracy of the lens 210.
Therefore, the object of the present invention is to provide a lens driving apparatus excellent in positional accuracy, preventing the adverse effect of rattling caused by the backlash and of excessively strong load imposed on the actuation of a driving source.
For achieving the above-mentioned object, the lens driving apparatus in accordance with the present invention is a lens driving apparatus for moving a lens along an optical axis, the lens driving apparatus comprising a lead screw rotated by a driving source; a lens holding frame for holding the lens, the lens holding frame being formed with a threaded part meshing with a threaded part of the lead screw and adapted to move along the optical axis as the lead screw rotates; a nut plate formed with a threaded part meshing with the threaded part of the lead screw and adapted to move along the optical axis as the lead screw rotates; rotation preventing means for preventing the lens holding frame and the nut plate from rotating as the lead screw rotates; and urging means for urging the lens holding frame and the nut plate in a direction by which the lens holding frame and the nut plate approach each other or separate from each other.
According to such a lens driving apparatus, the lens holding frame and nut plate are urged by the urging means in directions by which they approach each other or separate from each other, whereby each of the lens holding frame and nut plate moves by the amount of backlash generated with respect to the threaded part of the lead screw, so as to be biased at the meshing part. Therefore, positional accuracy of the lens driving apparatus is not adversely affected by rattling caused by the backlash.
Since the lens holding frame and nut plate are prevented from rotating as the lead screw rotates, the lens holding frame and nut plate move along the optical axis as the lead screw rotates, whereby the lens can accurately be moved along the optical axis.
Since each of the lens holding frame and nut plate meshes with the lead screw so as to move along the optical axis rotates while being prevented from rotating as the lead screw rotates, they move keeping the distance therebetween always constant as the lead screw rotates, whereby the urging action caused by the urging means is always held at a constant force. Therefore, the load imposed on the actuation of the driving source becomes constant, so that the rotation control of the driving source is not adversely affected, whereby the lens
Preferably, at least one of the respective threaded parts of the lens holding frame and nut plate is less than one pitch. In this case, even when there is an error in each pitch width of the lead screw, no friction/load occurs due to the error upon rotating the lead screw.
Also, forming the threaded part into less than one pitch simplifies the die cutting step when molding each member.
It further contributes to making the whole apparatus smaller.
Preferably, the urging means is a leaf spring. When the leaf spring is used, the urging means is formed thin, whereby the whole apparatus can be made smaller.
The lens is a focus lens for carrying out focusing in a taking optical system, for example.